A tearful television announcer dressed in black said the 69-year old had died Saturday of physical and mental over-work on his way to give "field guidance."
He had suffered a stroke in 2008, but appeared to have recovered. North Korea's official KCNA news agency said he died at 8:30 a.m. Saturday (6:30 p.m. EST on Friday) after "an advanced acute myocardial infarction, complicated with a serious heart shock."
South Korea, still technically at war with the North, placed its troops and all government workers on emergency alert, Yonhap news agency reported. But Seoul's Defense Ministry said there were no signs of any unusual North Korean troop movements.
"Up until tonight, if anybody had asked you what would be the most likely scenario under which the North Korean regime could collapse, the answer would be the sudden death of Kim Jong-il," said Victor Cha, a Korea expert with the Center for Strategic and International Studies think-tank in Washington.
"And so I think right now we're in that scenario and we don't know how it's going to turn out."
The White House said President Barack Obama had been notified of the reports of Kim's death and it was closely monitoring and in touch with South Korea and Japan.
The United States was committed to stability on the Korean peninsula as well as to its allies, the White House press secretary said.
Japan's Foreign Minister Jun Azumi said his country had to be prepared for the unexpected on the security front.